Mutations in the untranslated regulatory regions of genes may result in abnormal gene expression or transcriptional regulation. In this study, we characterize the ornithine transcarbamylase (OTC) mRNA isoforms of the X-linked OTC gene involved in the urea formation in the liver. Our data revealed that two major transcripts (OTC-t1 and OTC-t2) are more highly expressed than any of the other isoforms in all the tissues analyzed, though a longer transcript (OTC-t3) was also isolated and characterized from the brain sample. The OTC-t2 sequence fully matches the OTC mRNA reference sequence (NM_000531.5). All three isoforms use a canonical AAUAAA hexamer that is predicted to fold into a hairpin secondary structure which might be exposed to the cleavage and polyadenylation specificity factor. In addition, we observed that the OTC-t1 and OTC-t2 transcripts display heterogeneity at the cleavage sites in a tissue-dependent manner. Taken together, our data demonstrate that several mRNA isoforms are transcribed from the OTC gene, thereby indicating a wide degree of variability in post-transcriptional regulation.